The main point of quilting is not to reinforce seams. The main point of quilting is to keep the batting from shifting and lumping. If it says "up to 6 inches apart" they mean in all directions. Quilting close may or may not make a quilt stiff. That is also dependent on the batting you are using. Scrimmed battings, like Warm and Natural get stiff with more quilting. Needle punched battings don't get all that stiff with closer quilting. Some battings don't get stiff unless you're doing micro stippling.